Process fluid

ABSTRACT

A water-based, shear-thinning process fluid comprises bentonite, aluminium hydroxide particles, and salt. The median diameter (by weight) of the aluminium hydroxide particles does not exceed 2 μm.

FIELD OF THE INVENTION

The present invention relates to a water-based process fluid, such as adrilling fluid, and particularly a process fluid exhibitingshear-thinning behaviour.

BACKGROUND TO THE INVENTION

Thickened, water-based, process fluids which exhibit shear-thinningbehaviour are useful in a number of technical fields.

In particular for hydrocarbon well applications such fluids are used ase.g. drilling fluids, water control fluids and fracturing fluids. Atypical rheological requirement for the fluids is a reversible andsevere drop in viscosity when the shear rate imposed on the fluid isincreased.

Aqueous drilling fluids commonly contain clay particles and thickeningagents. For example U.S. Pat. No. 49,990,268 discloses a drilling fluidwhich contains negatively charged bentonite particles and positivelycharged mixed metal hydroxide particles and which is thickened by aheteroflocculation mechanism involving the formation of a gel networkbased on Coulombic (electrostatic) attraction between the bentonite andmixed metal hydroxide particles. A commercially available calcined mixedmetal hydroxide powder thickening agent supplied by M-I L.L.C. is soldunder the name Visplex™.

Clay-free drilling fluids containing viscosifying aluminium hydroxidecompounds are also known, as disclosed e.g. in U.S. Pat. Nos. 4,240,915,4,389,319, 4,349,443 and 4,486,318.

However, the stability of the rheological behaviour of these fluids maybe threatened by e.g. degradation at elevated temperatures, oxidationand/or the presence of contaminating salts, and an object of the presentinvention is to provide a water-based process fluid which exhibitsacceptable rheological behaviour over a wide range of operatingconditions.

SUMMARY OF THE INVENTION

In general terms, a first aspect of the present invention provides ashear-thinning, water-based process fluid comprising clay particles,colloidal particles, and salt.

In such a fluid at 20° C. (and preferably also at higher temperatures),primarily van der Waals interactions between the clay and colloidalparticles can organise the clay and colloidal particles into a gelnetwork which thickens the fluid. These interactions can be broken andreimposed as the shear rate experienced by the fluid respectivelyincreases and decreases, leading to reversible shear-thinning behaviour.

The process fluid may be e.g. a drilling fluid, water control fluid,fracturing fluid, or other treatment fluid for use in a hydrocarbonwell.

A primary purpose of the salt is to form ions in solution whichelectrostatically screen the clay and the colloidal particles. Thisprevents or reduces electrostatic interactions between the particles sothat van der Waals interactions can be established. Thus by “salt” wemean any soluble compound or compounds which is/are able to provide suchions. Preferably the salt comprises NaCl and/or KCl. In a process fluidsuch as a sea water-based drilling mud these salts can be provided bythe sea water. However, if salts of higher-valency cations are used,these will be effective at lower concentrations than NaCl or KCl,following the Schulze-Hardy rule for the screening of double layerrepulsions.

Thus, whereas known drilling fluids which are primarily thickened byelectrostatic attractions between particles (e.g. bentonite base mudscontaining Visplex) are sensitive to salt contamination (we believe thesalt reduces the efficiency of the heteroflocculation mechanism bypromoting aggregation of like particles, thus disturbing desirableelectrostatic attractions and reducing the amount of thickening), theprocess fluid of the present invention is generally insensitive to orunaffected by such contamination because most salts do not reduce thevan der Waals attractions between the particles. Indeed the processfluid of the present invention includes salt. This can be a significantadvantage e.g. in off-shore hydrocarbon well applications where it isoften convenient to make up process fluids from sea water.

The fluid may further comprise a fluid loss control agent, such asFloplex™, IdFlo™ (both supplied by M-I L.L.C.) or other modified starchbased agent for reducing fluid loss during well drilling operations.Like salt additions, these types of agent tend to decrease theeffectiveness of process fluids in which thickening is caused byelectrostatic attractions between particles. However, we have found thatthey do not, in general, degrade the performance of fluids according tothe present invention. In particular, because fluids according to thepresent invention already contain salt, they are relatively insensitiveto further additions of charged species. Thus it becomes possible to usefluid loss control agents which form charged species in solution andtherefore would have downgraded the rheological performance ofconventional drilling fluids.

The clay particles may be of bentonite. Preferably, the clay particles(which are generally in the form of platelets) have an average diameterin the range 0.1 to 2 μm and/or an average thickness in the range of0.01 to 0.001 μm.

The colloidal particles may be formed from any suitable compound(s)which promotes van der Waals interactions between the clay and colloidalparticles. Thus the colloidal particles may be of metal oxides and/orhydroxides, which have in general relatively large Hamaker constantscharacterising the interactions between macroscopic particles in liquids(see e.g. chapter 11 and table 11.2 of J. N. Israelachvili:“Intermolecular and surface forces”, 2^(nd) edition, Academic Press,London, 1992) and therefore promote van der Waals attractions. Forexample, the particles may be of aluminium, magnesium and/or zirconiumoxide and/or hydoxide. Preferably the particles include aluminiumhydroxide compounds.

In one embodiment, the median or mean diameter (by weight) of thecolloidal particles does not exceed 2 μm, and preferably does not exceed1 μm. By limiting the size of the colloidal particles, the particlesmore readily form a colloidal suspension in the fluid. Also theirweight-efficiency for thickening the fluid increases.

Where it pertains to colloidal particles, use herein of the term“diameter” does not imply that the colloidal particles are required tobe spherical. The term is to be understood as encompassing thespherical-equivalent diameter of non-spherical colloidal particles aswell as the “true” geometrical diameter of spherical colloidalparticles, as the case may be.

Clay and colloidal particle sizes may be measured by transmissionelectron microscopy (TEM). Alternatively or additionally they may bemeasured indirectly from sedimentation rates determined byultra-centrifuge experiments (see e.g. J. L. Cole and J. C. Hansen,Journal of Biomolecular Techniques, 10, (1999), 163). This technique isparticularly suitable for measuring the spherical-equivalent diameter ofnon-spherical colloidal particles.

For the interpretation of sedimentation rates, the sedimentationcoefficient, s, is given by the slope of ln(r(t)-r(t0)) versus Ω²t;where r(t) is the amount of sedimentation (as measured e.g. byabsorbance) at a radial distance r from the center of rotation at a timet, and Ω is the angular velocity of the ultra-centrifuge. However, s, isalso defined in the Svedberg equation (T. Svedberg and K. O. Pedersen,The Ultracentrifuge, Theodor Steinkopff, Dresden, Germany, 1940) bys=VΔρ/f, where V (which equals (4/3)πR³) is the volume of the particle,Δρ is the mass density difference between the particle and the solvent,and f is the Stokes friction factor equal to 6πηR (R being the radius ofparticles in the case of spherical particles or the sphere-equivalentradius in the case of non-spherical particles, and η being the solventviscosity). Thus, R (and hence the particle diameter, 2R) can bedetermined from a measured value for s, and knowledge of Δρ and η. Usingthe ultra-centrifuge technique average particle diameters and particlediameter distributions can be determined.

Thus one preferred embodiment of the present invention provides awater-based, shear-thinning process fluid comprising bentonite,aluminium hydroxide particles, and salt, wherein the median or meandiameter (by weight) of the aluminium hydroxide particles does notexceed 2 μm.

The fluid may have a pH at or above the isoelectric point of thecolloidal particles. For example, aluminium hydroxide particles (asexemplified by gibbsite or boehmite) have an isoelectric point of aboutpH 9.5 (A. Wierenga et al., Colloids Surfaces A, Vol. 134, (1998), 359).Thus, if the particles are of aluminium hydroxide, the pH of the fluidis preferably in the range 9.5-11. Generally clay particles carry anegative charge in aqueous suspension at pH 7 and above, and by imposinga pH at or above the isoelectric point the colloidal particles will lackan opposite charge. This has the effect of further promoting van derWaals over electrostatic attractions.

Preferably the colloidal particles are non-spherical. More preferablythey have a platelet (e.g. as in gibbsite) and/or rod-like (e.g. as inboehmite) morphology. In terms of promoting gel networks based on vander Waals interactions, these morphologies seem to be compatible withthe plate-like shape of most clay (and particularly bentonite)particles. Also, we believe the larger surface area of non-sphericalparticles promotes van der Waals interactions.

In one embodiment a surfactant is provided on the surfaces of thecolloidal particles. Particles with surfactant appear to be more easilydispersed when formulating the fluid. We believe that, by reducinginter-colloidal particle aggregation and enhancing clay-colloidheteroflocculation, they also increase the amount of thickening ofparticularly bentonite-containing process fluids.

Preferably the concentration of the colloidal particles in the fluid isin the range 0.5 to 6 g/l, and more preferably in the range 1 to 4 g/l.Preferably the concentration of the clay particles in the fluid is inthe range 5 to 60 g/l, and more preferably in the range 15 to 40 g/l.Preferably the concentration of the salt in the fluid is in the range 5to 150 g/l, and more preferably in the range 10 to 80 g/l.

The process fluid may have a 10″ gel strength at 20° C. in the range 15to 70 Pa. By ‘10″ gel strength’ we mean the stress measured at a shearrate of 5 s⁻¹ after a 10 second shear-free period.

Preferably the process fluid maintains a 10″ gel strength at 20° C. inthe range 15 to 70 Pa after ageing at a pressure of 1.7 MPa forconsecutive periods of 16 hours at 240° F. (115.6° C.) and 16 hours at300° F. (148.9° C.).

The process fluid may exhibit reversible shear-thinning behaviour at 20°C. for all shear rates in the shear rate range 5 to 1000 s⁻¹.

The process fluid may have a rate of change of viscosity with shear rateat 20° C. of between −10 and −0.01 Pa·s for all shear rates in the shearrate range 5 to 1000 s⁻¹.

Such performance characteristics suggest that the fluid has rheologicalproperties suitable for hydrocarbon well applications.

In a further aspect, the present invention provides for the use of awater-based process fluid of the previous aspect for drilling ortreating a hydrocarbon well.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the following drawings in which:

FIG. 1 shows a TEM micrograph of typical gibbsite particles.

FIG. 2 shows typical flow curves of example and comparative examplesystems.

FIG. 3 shows the influence of NaCl concentration on the 10″ gel strengthof an aqueous fluid containing 33 g/l of bentonite and 3.3 g/l ofcolloidal gibbsite.

FIG. 4 shows the 10″ gel strengths at 20° C. for fluids before and afterthe addition of a 0.02 M PO₄ ²⁻ concentration in each fluid.

FIG. 5 shows the effect on the 10″ gel strength at 20° C. for exampleand comparative example systems.

FIG. 6 shows the API fluid loss and 10″ gel strengths at 20° C. forfluids before and after the addition of an amount of fluid loss controlagent.

FIGS. 7 a-c show respective electron micrographs of M111, M107 and BN-2powders.

FIGS. 8 a-c show respective electron micrographs of AS 100, AS 200 andAS 520 powders.

FIG. 9 shows an electron micrograph of MH-1 powder.

FIGS. 10 a-c show values of the yield points, 10″ gel strengths at 20°C. and plastic viscosities of various muds. FIG. 10 a shows these valuesfor muds containing 28 g/l of bentonite and different amounts of M111;FIG. 10 b shows these values for muds containing 33 g/l of bentonite anddifferent amounts of M107; and FIG. 10 c shows these values for mudscontaining 33 g/l of bentonite and different amounts of BN-2.

FIGS. 11 a-d respectively show flow curves for four mixtures before hotrolling (BHR) and after hot rolling (AHR) at ageing temperatures of 250,300 and 350° F., the mixtures containing (a) 28 g/l bentonite and 1.2g/l M111, (b) 33 g/l bentonite and 3.4 g/l M107, (c) 33 g/l bentoniteand 5.6 g/l BN-2, and (d) 33 g/l bentonite and 3.3 g/l GW3A.

FIG. 12 shows the 10″ gel strengths at 20° C. of the mixtures of FIGS.11 a-d as a function of the consecutive ageing temperatures.

FIG. 13 shows the results of API fluid loss tests performed on the mudsof FIGS. 11 a-c, and the results of further tests after the addition of11 g/l of Floplex to each mud. The Figure also shows the result an APIfluid loss test performed on the mud of FIG. 11 d after an addition of11 g/l of Floplex.

FIG. 14 shows flow curves before static ageing (BSA) for threeAluminasol-containing muds, and the M111 and GW3A muds of FIGS. 11 a andd.

FIG. 15 shows flow curves for the AS 520 mud of FIG. 14 before staticageing (BSA) and after static ageing (ASA) at consecutive temperaturesof 250, 300 and 350° F.

FIG. 16 show a flow curve for a mud containing 33 g/l of bentonite and10 g/l of MH-1.

FIG. 17 shows an electron micrograph of rod-like colloidal particlesformed as a result of hydrothermal treatment of ACH.

FIG. 18 shows flow curves for a 33 g/l bentonite mud with and without insitu precipitated AL(OH)₃.

DETAILED DESCRIPTION

Example systems of bentonite-containing aqueous fluids according to thepresent invention were produced. These were compared in rheological andfluid loss tests with comparative example systems. Thereafter, a numberof gibbsite and boehmite samples were sourced from commercial suppliers,and their suitability for use in the present invention was tested.

EXAMPLE SYSTEMS

Example systems were produced containing bentonite, NaCl and eithergibbsite or boehmite colloidal particles. The gibbsite and boehmiteparticles were formed in aqueous suspension from fine chemicals asdescribed e.g. by A. Wierenga et al., Colloids Surfaces A, Vol. 134,(1998), 359, A. P. Philipse et al. Langmuir, Vol. 10, (1994), 4451, andF. M. van der Kooij et al., J. Phys. Chem. B, Vol. 102, (1998), 7829.The average largest dimension of the (plate-like) gibbsite particles wasaround 200 nm (FIG. 1 shows a TEM micrograph of typical gibbsiteparticles). The average largest dimension of the (rod-like) boehmiteparticles was also around 200 nm. Thus the median diameter of thecolloidal particles did not exceed 1 μm.

Example System 1 contained 33 g/l of bentonite, 3.3 g/l of plate-likegibbsite and 15 g/l of NaCl.

Example System 2 contained 33 g/l of bentonite, 5.4 g/l of rod-likeboehmite and 15 g/l of NaCl.

Example System 3 contained 33 g/l of bentonite, 3.9 g/l of plate-likegibbsite and 15 g/l of NaCl.

COMPARATIVE EXAMPLE SYSTEMS

Comparative System 1 was a simple reference slurry containing 38 g/l ofbentonite.

Comparative System 2 contained 28 g/l of bentonite and 3.3 g/l ofproprietary (Visplex) calcined mixed metal hydroxide powder.

Comparative System 3 contained 33 g/l of bentonite and 4.2 g/l of anearlier form Visplex which was produced by a chemical synthetic routeand contained an amount of glycerol.

Comparative System 4 contained 33 g/l of bentonite and 3.3 g/l ofcalcined Visplex.

Comparative System 5 contained 33 g/l of bentonite and 3.6 g/l ofcalcined Visplex.

Calcined Visplex is predominantly formed of mixed metal hydroxide (seee.g. U.S. Pat. No. 49,990,268) and has a mean particle size of at least4 μm which is expected to be too large for the formation of a colloidalsuspension. The predominant interaction between the bentonite andVisplex particles is believed to be electrostatic. Aqueous fluidscontaining bentonite and Visplex have been used commercially as drillingfluids.

Rheological Tests

These were performed on a Chan 35™ rheometer. The pH of the ExampleSystems and Comparative Systems was adjusted to 10.5 before rheologicalmeasurements were made. The pH was chosen in order to favour van derWaals interactions between the bentonite and colloidal aluminiumhydroxide particles of the Example Systems.

FIG. 2 shows typical flow curves measured in a shear sweep at 20° C. forExample Systems 1 and 2 and Comparative Systems 1 and 2. Shear-thinningbehaviour was shown by all the fluids, but as expected the fluidscontaining gibbsite, boehmite and Visplex additions were significantlythicker than the reference fluid containing only bentonite. ExampleSystems 1 and 2 and Comparative System 2 had viscosities of the order ofmagnitude needed for drilling fluid applications.

It was noted that the 10″ gel strength at 20° C. for Example Systems 1and 2 was always close to the 5 s⁻¹ stress measured in the shear sweepof FIG. 2. This indicates that the gel networks of these fluids areformed almost instantaneously. Thus if used as drilling fluids thesefluids should adapt rapidly to local shear rate conditions.

FIG. 3 shows the influence of NaCl concentration on the 10″ gel strengthat 20° C. of an aqueous fluid containing 33 g/l of bentonite and 3.3 g/lof colloidal gibbsite. The maximum gel strength is reached at about 18g/l of NaCl (in a similar test using colloidal boehmite instead ofgibbsite the maximum was at about 15 g/l of NaCl). Thus these fluidsattain their maximum gel strengths at salt concentrations similar tothose found in sea water (sea water is essentially 0.5 M NaCl=14 g/l),and could conveniently be formed from sea water in offshore drillingapplications.

Evidence of the difference in mechanism underlying gel network formationin the case of the Example Systems and the Comparative Systems isprovided by the effect of additions of low concentrations of phosphateanions. FIG. 4 shows the 10″ gel strengths at 20° C. for Example System3 and Comparative Systems 3 and 4 before and after the addition of a0.02 M PO₄ ³⁻ concentration in each fluid. The screening of the positivecharge on the Visplex particles by the trivalent anions led to adramatic loss in gel strength of the Comparative Systems, while thestrength of the gel network in Example 3 was practically unimpaired.This insensitivity of gel networks produced by van der Waalsinteractions to anionic contamination should offer substantialadvantages in hydrocarbon well applications.

Next the thermal stabilities of various fluids were investigated.Example Systems 1 and 2 and Comparative Systems 1 and 2 were eachsubjected to 16 hour periods at temperatures of 240, 300, 350 and 375°F. (115.6, 148.9, 176.7 and 190.6° C.) in rolled autoclave bombsprepressurised to 1.7 MPa. Between each ageing period each fluid washomogenised and had its pH readjusted to 10.5 before its 10″ gelstrength at 20° C. was measured.

FIG. 5 shows the results of the ageing tests. The ability of ComparativeSystem 2 to form a gel network was clearly compromised by ageing above240° F. In contrast, Example Systems 1 and 2 were thermally stable fortemperatures up to at least 375° F.

This thermal stability can be explained by the fact that the gel networkgenerated by van der Waals interactions only requires the presence ofsmall particles (i.e. the gibbsite and boehmite) to bridge thebentonite—a condition which is less likely to be threatened bytemperature-induced conversion than the particle surface charge requiredto form the gel network in the Visplex-based fluids.

Fluid Loss Tests

API RP 13B fluid loss tests (see API Recommended Practice StandardProcedure for Testing Drilling Fluids, 8^(th) Edition, 1980, AmericanPetroleum Institute, Washington D.C.) were performed on Example Systems1 and 2 and Comparative System 5. The results of the tests are shown inFIG. 6. The fluid losses of the Example Systems were comparable withthose of the Comparative System.

The tests were then repeated after 11 g/l of Floplex fluid loss controlagent had been added to each system. Again the fluid losses of theExample Systems were comparable with those of the Comparative System.However, the 10″ gel strengths at 20° C. of the Example Systems wereincreased by the Floplex additions, while the 10″ gel strength at 20° ofthe Comparative System was reduced. This further demonstrates theability of the process fluids of the present invention to maintaindesirable rheological characteristics under a range of conditions.

Commercial Gibbsite and Boehmite Samples

Next, gibbsite and boehmite samples obtained from several commercialsuppliers were investigated.

Martinswerk GmbH (PO Box 12 09, D-50102 Bergheim, Germany)

Three powder samples were received from this supplier: Martinal OL 107(M107), Martinal OL 111/LE (M111), both specified by the supplier asfine precipitated aluminium tri-hydrates (Al(OH)₃) with plate-likecrystal moropholgies, having median particle sizes of approximately 0.7μm (M111) and 1.5 μm (M107) (as determined by the supplier using lightscattering); and Martoxal BN-2 (M BN-2), a boehmite (AlOOH) with amedian particle size of approximately 1 μm. The specific surface areasof the three samples are in the range 3 to 20 m²/g. By a combination ofTEM and ultracentrifuge measurements we confirmed the presence ofparticles having diameters slightly below 1 μm in M111 and slightlyabove 1 μm in M107 and BN-2. The particles of the M111 material containa small amount of polyacrylate surfactant to facilitate redispersion,which makes the surface anionic at neutral pH. At the pH of 10.5 adoptedthroughout this study, these particles are anionic as are the gibbsiteand boehmite particles of samples at that pH which do not containsurfactant. FIGS. 7 a-c show respective electron micrographs of theM111, M107 and BN-2 powders.

Nissan Chemical Industries, Ltd. (7-1, 3-chome, Kanda-Nishili-cho,Chiyoda-ku, Tokyo, Japan 10)

Three samples were received from this supplier: Aluminasol (AS) 100, 200and 520, all three being in the form of colloidal suspensions. TEM andultracentrifuge measurements showed that the AS 200 and AS 520 particleswere about 50 nm in diameter, whereas the AS 100 particles were about150 nm in diameter. FIGS. 8 a-c show respective electron micrographs ofthe AS 100, AS 200 and AS 520 powders.

Malakoff Industries Inc. (PO Box 457, Malakoff, Tex. 75148-0487, USA)

One sample of powdered gibbsite was received from this supplier: MH-1.The median particle size was specified as being 3 μm. This was confirmedby TEM observations. FIG. 9 shows an electron micrograph of the MH-1powder.

For comparison with these commercial samples, a gibbsite suspension(designated GW3A) was prepared in a similar manner to the gibbsite usedin the Example Systems discussed above.

Rheology, Thermal Stability and Fluid Loss of Muds Containing M111, M107and 3N-2

Each of M111, M107 and BN-2 was mixed as a dry powder to a bentonitebase and then dispersed in water to form a mud. Further muds were formedby varying the amounts of the M111, M107 and BN-2 powders in themixtures. Each mud was then rheologically and thermally tested in asimilar manner to the Example and Comparative Systems described above(i.e. subjected to consecutive 16 hour periods of hot rolling attemperatures of 240, 300, 350° F. with rheological testing beingperformed before thermal ageing and between each period).

FIGS. 10 a-c show values of the yield points (the shear stress at zeroshear rate, calculated by extrapolation of measured shear stress vs.shear rate flow curves to zero shear rate), 10″ gel strengths andplastic viscosities (the slope of the measured shear stress vs. shearrate curve at the highest measured shear rates) at 20° C. of variousunaged muds. FIG. 10 a shows these values for muds containing 28 g/l ofbentonite and different amounts of M111; FIG. 10 b shows these valuesfor muds containing 33 g/l of bentonite and different amounts of isM107; and FIG. 10 c shows these values for muds containing 33 g/l ofbentonite and different amounts of BN-2.

Although all three Martinswerk products do increase the gel- and shearthinning characteristics of bentonite mud, the M111 agent is the mostreproducible and effective among them. We believe this reproducibilityis due to the surfactant which is present only on the M111-particles,and which facilitates dispersion of the powdered material into asuspended colloid. The less strong correlation between thickenerconcentration and gel strength in the case of M107 and BN-2 may becaused by the poorer dispersability of these surfactant-free powders.

A surprisingly low concentration of M111 material is required to producea thickening effect in the bentonite mud. We believe this is alsoconnected with the surfactant present on the M111 particles: thesurfactant allowing heteroflocculation between M111 and bentoniteparticles while opposing mutual aggregation between M111 particles. Thatis, the surfactant apparently promotes selective inter-particlebridging.

Flow curves, measured in shear sweeps at 20° C., for four mixturesbefore hot rolling (BHR) and after hot rolling (AHR) at ageingtemperatures of 250, 300 and 350° F. are shown respectively in FIGS. 11a-d, the mixtures containing (a) 28 g/l bentonite and 1.2 g/l M111, (b)33 g/l bentonite and 3.4 g/l M107, (c) 33 g/l bentonite and 5.6 g/lBN-2, and (d) 33 g/l bentonite and 3.3 g/l GW3A. Mixture (d) is similarto Example System 1 discussed above.

For all of the muds, ageing up to 300° F. led to a decrease in stress atmedium and high shear rates compared to the stress BHR, while leavingthe low shear rate gel characteristics substantially unchanged. Thus themeasured stress profiles became flatter than before ageing, whichsuggests that wall-slip could be affecting these measurements.

The low shear rate stresses are several times higher than the shearstress measured for the unaged bentonite base mud without thickener (seeFIG. 2). Combined with the very low plastic viscosities observed for theaged muds, this indicates that the muds would make useful drilling muds(i.e. the shear thinning behaviour is conserved after ageing attemperatures of up to 300° F.).

A general characteristic of the behaviour of all the muds, except forthe BN-2 boehmite mud, is an increase in stress at all shear rates afterthe final ageing step at 350° F. However, from FIG. 5 we expect thistrend to be reversed again at even higher temperatures/ageing times, andtherefore we do not regard this as a threat to practical use.

The 10″ gel strengths at 20° C. of these, muds is shown in FIG. 12 as afunction of the consecutive ageing temperatures.

API fluid loss tests were performed on the M111, M107 and BN-2 muds, andthen re-performed after the addition of 11 g/l of Floplex to each mud.An API fluid loss test was also performed on the GW3A mud after anaddition of 11 g/l of Floplex. The results of the tests are shown inFIG. 13. With Floplex added, the fluid loss of the M111 mud was slightlyhigher than that of the GW3A mud, but the fluid losses of all the mudswith Floplex added were comparable to those shown in FIG. 6. It wasnoted that the addition of Floplex also gave rise to a moderate increasein the gel strengths of the muds.

Rheology and Thermal Stability of Muds Containing AS 100, AS 200 and AS520

Three muds containing 33 g/l of bentonite and 5.4 g/l of respectively AS100, AS 200 and AS 520 were prepared. Because the pH of the Aluminasolsuspensions was low (pH is about 4) to ensure the stability of thepositively charged particles in the suspensions, care had to be taken toprevent the bentonite mud collapsing at low pH values when theAluminasol suspensions were added to the mud. Therefore each suspensionwas added in a sequence of small additions, with pH readjustment of themud between each addition. Each mud was then rheologically and thermallytested in a similar manner to Martinswerk samples, except that (i) aBohlin™ rheometer with a Couette geometry was used instead of the Chanrheometer, and (ii) the thermal ageing was performed without rolling ofthe autoclave bombs.

FIG. 14 shows flow curves, measured in shear sweeps at 20° C., for thethree Aluminasol-containing muds and, for comparison, the M111 and GW3Amuds previously described. Of the Aluminasol muds, only the onecontaining AS 520 had rheological characteristics comparable to themodel gibbsite GW3A and M111 muds. The poorer performance of AS 100 andAS 200 does not seem to be connected with their particle sizes, whichare practically equal to that of AS 520. It seems more likely that thedifference in performance stems from unidentified chemical additiveswhose presence was indicated by ultra centrifuge measurements on theAluminasol suspensions.

The stability of the rheological behaviour of the AS 520 mud was testedas a function of ageing history. FIG. 15 shows flow curves, measured inshear sweeps at 20° C., for the AS 520 mud before static ageing (BSA)and after static ageing (ASA) at consecutive temperatures of 250, 300and 350° F. The flow curves were more or less stable with ageing up to300° F., but the shear stress reduced dramatically after the last ageingstep at 350° F. This may be due to the unidentified chemical additivesin the AS 520 suspension.

Rheology of a Mud Containing MH-1

MH-1 was mixed as a dry powder to a bentonite base and then dispersed inwater to form muds containing up to 10 g/l of MH-1 and 33 g/l ofbentonite. The muds were then rheologically tested in a similar mannerto Martinswerk samples. Thermal ageing was not performed.

FIG. 16 show the flow curve, measured in a shear sweep at 20° C., forthe mud with 10 g/l of MH-1. Although the MH-1 concentration was variedup to 10 g/l, the Malakoff gibbsite did not thicken the 33 g/l bentonitemud. we believe the reason for this was the size of the MH-1 particles,which was too large for the gibbsite to form a colloidal suspension inthe mud. This prevented a gel network mediated by van der Waalsinteractions between the bentonite and MH-1 particles from being formed.

Synthesis of Gibbsite

Synthesis routes for the preparation of gibbsite were investigated toprovide alternatives to the commercially available gibbsites andboehmites discussed above.

An amount of Al₂(OH)₅Cl.3H₂O (ACH) was received from Hoechst under thename of Locron P™. ACH solutions were treated hydrothermally for 24-96hours at 85° C. FIG. 17 shows an electron micrograph of the rod-like(which is characteristic of boehmite) colloidally suspended particleswhich were formed as a result of the hydrothermal treatment. Laser beamscattering indicated that longer treatment periods led to higherconcentrations of the particles.

However, after hydrothermal treatment the concentration of Al³⁺ insolution was still very high and Al(OH)₃ precipitated when the solutionwas brought to pH 10.5.

Thus two different methods were investigated for the formulation ofbentonite-base muds. In the first method, the pH of an ACH solution wasadjusted to pH 10.5 prior to its addition to a bentonite-base mud. Thisled to Al(OH)₃ precipitation before contact with the mud. In the secondmethod, ACH powder was added directly to the alkaline bentonite mud,leading to in situ Al(OH)₃ precipitation.

The first method yielded particles which were colloidal-sized (about 150nm according to ultra-centrifuge measurements) and could be expected tobe rather poorly defined in shape. Visual inspection of a mud containing35 g/l bentonite and 5 g/l ACH-colloid mixture indicated that excellentrheological behaviour was obtained.

In the second method, the addition of the ACH powder gave rise toinstantaneous gelation of the bentonite mud. We believe that the Al³⁺precipitated immediately, probably as amorphous Al(OH)₃ particles ofundefined shape. In situ preparation of Al(OH)₃ particles may havepractical advantages as a method of thickening drilling fluids.

FIG. 18 shows flow curves, measured in shear sweeps at 20° C., for a 33g/l bentonite mud with and without in situ precipitated Al(OH)₃ formedby the addition (to the mud at pH 10.5) of 5 g/l of ACH.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1. A water-based, shear-thinning process fluid comprising: bentonite,aluminium hydroxide particles, and salt, the median diameter by weightof the aluminium hydroxide particles not exceeding 2 μm, and the pH ofthe process fluid is in the range of 9.5 to
 11. 2. A process fluidaccording to claim 1, wherein the median diameter of the aluminiumhydroxide particles does not exceed 1 μm.
 3. A process fluid accordingto claim 1, wherein the salt comprises NaCl and/or KCl.
 4. A processfluid according to claim 1, wherein the aluminium hydroxide particlesare non-spherical.
 5. A process fluid according to claim 4, wherein thealuminium hydroxide particles are platelets and/or rods.
 6. A processfluid according to claim 1 comprising a surfactant on the surfaces ofthe aluminium hydroxide particles.
 7. A process fluid according to claim1, wherein the aluminium hydroxide particles in the fluid have aconcentration in the range 0.5 to 6 g/l.
 8. A process fluid according toclaim 1, wherein the bentonite in the fluid has a concentration in therange 5 to 60 g/l.
 9. A process fluid according to claim 1, wherein thesalt in the fluid has a concentration in the range 5 to 150 g/l.
 10. Aprocess fluid according to claim 1 further comprising a fluid losscontrol agent.
 11. A process fluid according to claim 1, having a 10″gel strength at 20° C. in the range 15 to 70 Pa.
 12. A process fluidaccording to claim 11, which maintains a 1041 gel strength at 20° C. inthe range 15 to 70 Pa after ageing at a pressure of 1.7 MPa forconsecutive periods of 16 hours at 240° F. (115.6° C.) and 16 hours at300° F. (148.9° C.).
 13. A process fluid according to claim 1 whichprovides exhibiting reversible shear-thinning behaviour at 20° C. forall shear rates in the shear rate range 5 to 1000 s⁻¹.
 14. A processfluid according to claim 1, having a rate of change of viscosity with ashear rate at 20° C. of between −10 and −0.01 Pa·s for all shear ratesin the shear rate range 5 to 1000 s⁻¹.
 15. A process fluid according toclaim 1 which is a drilling fluid.
 16. In a process of drilling ahydrocarbon well, of the type wherein a process fluid havingshear-thinning behavior is used as a drilling fluid, water controlfluid, fracturing fluid, or a combination thereof, the improvementcomprising the step of using the water-based process fluid according toclaim 1 as the drilling fluid, water control fluid, or the fracturingfluid, either alone or in combination.